The personality profiles for youths with Prader-Willi, fragile-X, or Williams syndrome were compared to three matched groups attending regular schools. Using the California Child Q-Set (CCQ), both of the parents of the 39 children with Prader-Willi syndrome, 32 boys with fragile-X syndrome, 28 children with Williams syndrome, and children in the comparison groups provided independent personality descriptions in terms of the Big Five personality factors of Extraversion, Agreeableness, Conscientiousness, Emotional Stability, and Openness, along with Motor Activity and Irritability. Specific personality phenotypes for each of the three syndrome groups were found to be differentially related to parental behaviours (i.e. control and anger) and family contexts (i.e. experienced family stress, marital conflict, and parental consistency).